1. Field of the Invention
The invention pertains to a landing assistance system for aircraft.
2. Description of the Prior Art
The most recent landing assistance system, which is a standardized one, is a microwave landing system called MLS. It has two narrow microwave beams which do a vertical scanning and a horizontal scanning, respectively, in the direction of a landing strip. These two beams are used to determine the position of an aircraft in a vertical plane and a horizontal plane respectively. This MLS system is currently installed or will be installed in all big airports, but has the disadvantage of requiring complicated infrastructure for each landing strip. This infrastructure is, firstly, too expensive for small airports and, secondly, too complicated to be set up quickly, for tactical needs, at a military landing strip.
Furthermore, there are very precise known positioning systems that use navigation satellites, for example the positioning system called NAVSTAR/GPS. A system of this type comprises several satellites. Each satellite emits signals controlled by an atomic clock and comprising, notably, almanac data which make it possible to ascertain the position of the satellite with respect to a geodesic reference point. A positioning receiver, placed on board an aircraft for example, is used to determine the position of this aircraft by simultaneously receiving the signals emitted by several satellites located in direct line of view at a given moment. A constellation of eighteen satellites is provided for, in order to enable reception from four or five satellites at any instant at any point on the earth. Each positioning receiver has a computing device, used to compute the position of the aircraft in three dimensions at high speed, using all the cumulated phase data of the carrier emitted by each satellite and using the phase of a code modulating this carrier.
A differential method described, for example, in the French Journal NAVIGATION, No. 137, pages 88 to 91, Jan. 1987, makes it possible to improve the positioning precision achieved through the GPS system. By this differential method, the precision obtained is of about 3 meters on an average and makes it possible to envisage the use of the GPS system as an aircraft landing assistance system. According to this differential method, the system comprises a fixed station which constitutes a reference for aircraft located in the vicinity of the said fixed station, within a radius of 100 kilometers for example. The fixed station has a receiver for positioning by satellite, which gives an estimate of the position of this fixed station, and computing means that give correction data representing the deviation between this estimate and a reference position, which may be the position of the fixed station identified on a map. After eliminating the essential part of the lag of the clock incorporated in the receiver of the fixed station, the distances observed for each satellite, called pseudo-distances, are compared with the exact differences calculated from the almanac data transmitted by the satellites and from the reference position. The result of these computations give correction data, used to compensate for most of the errors arising out of satellite ephemerides and out of uncertainties in the propagation of radio waves.
The fixed station further comprises radio transmission means to transmit the correction data to aircraft. The station on board each aircraft comprises, in addition to the receiver for positioning by satellite, radio reception means to receive the correction data and computing means for correcting, by means of the correction data, the position estimate given by the positioning receiver. This computing device may be incorporated in the positioning receiver, which is then called a differential positioning receiver and directly gives the value of the corrected estimate.
The corrected estimate is precise enough to be used to guide a landing operation, but requires additional reception equipment. However, for reasons related to space factor and cost, it is not desirable to increase the quantity of equipment on aircraft so that they can use another landing assistance system that complements the MLS system.